1. Field of the Invention
This invention relates to a new illumination apparatus for a copying machine. More particularly, the invention relates to a new and novel apparatus for controlling the intensity, i.e., the "darkness" or "lightness", of the copy produced by the machine, as well as the wavelength of the light radiated to the photoconductor element of the copying machine. The invention finds particular utility in applications where the copying machine experiences rough, continuous or near continuous service in a variety of environments, as, for example, a copying machine used at an on-site location to copy long, continuous original documents having variable optical permeability characteristics, such as well logs or seismographic tapes.
2. Description Of The Prior Art
A copying machine typically requires a high power lamp to illuminate the original document being copied. A fundamental problem associated with copying long, continuous documents, e.g., well logs, seismographic tapes, or cardiopulmonary records, is how to avoid the heat buildup that accompanies continuous operation of the high power lamp. Moreover, even in intermittent service, a typical incandescent lamp must be positioned far enough away from the original document to prevent scorching or igniting the original due to the heat generated by the lamp. Since the intensity of the light reflected on or through the original document is inversely proportional to the square of its distance from the light source, the spacing requirement of the incandescent lamp means less light will reach the document unless the power of the lamp is increased accordingly. But as the power of the lamp is increased, so does the amount of heat it generates.
One solution suggested by the art is the use of a fluorescent lamp. For a given candlepower, fluorescent lamps generate much less heat than incandescent lamps. Thus, not only can a fluorescent lamp be used continuously without undue heat buildup, but it can be used in close proximity to the original document without damaging the document, which results in more efficient use of the lamp and a more compact design for the copying machine.
Fluorescent lamps, however, present their own peculiar problems, especially where it is desirable to provide a copying machine with the capability of making copies of different degrees of darkness and, thus, the capability of making copies of original documents which are different in their optical permeability. For example, fluorescent tubes, the specific form of fluorescent lamps typically utilized in copying machines, show a marked decrease in light intensity at their ends. To remedy this problem, Nishikawa, U.S. Pat. No. 4,248,517, taught the use of a fluorescent tube mask having nonuniform, circumferential openings for producing a uniform light distribution.
Another distinct problem associated with fluorescent lamps is their inherent inflexibility. The intensity of the light produced by the lamp in a copying machine largely determines the "lightness" or "darkness" of the copy produced, so a range of intensities is desirable. The light intensity of a halogen lamp or an incandescent lamp can be changed simply by varying the voltage input to the lamp. Varying the voltage input to a fluorescent lamp, however, only minimally affects the light intensity.
Moreover, the wavelength of the light produced by a fluorescent lamp is similarly unaffected by varying the voltage input to the lamp. Since the spectral response curve of the photoconductor element of a copying machine varies depending on the type of photoconductor material used to coat the photoconductor element, and since even for a given photoconductor material the material will, in general, function optimally only within a given range of wavelengths, it is accordingly advantageous to be able to tailor the wavelength of the light from the fluorescent lamp to the operating characteristics of the photoconductor material. Due to continuing advances in photoconductor technology, it is also possible that existing photocopying machines may be upgraded by simply changing the photoconductor element of the machine. It is therefore particularly advantageous to provide the copy machine with an illumination apparatus that can readily be retrofitted or adapted to operate compatibly with various photoconductor materials.